The NADP(+)-isocitrate dehydrogenase gene (icd) is nitrogen regulated in cyanobacteria

NADP(+)-isocitrate dehydrogenase (NADP(+)-IDH) activity and protein levels in crude extracts from the unicellular cyanobacterium Synechocystis sp; strain PCC 6803 and the filamentous, dinitrogen-fixing Anabaena sp, strain PCC 7120 Here determined under different nitrogen conditions. The highest NADP(+)-IDH activity and protein accumulation were found under dinitrogen-fixing conditions for the Anabaena strain and under nitrogen starvation for Synechocystis sp. PCC 6803. The icd gene that encodes the NADP(+)-IDH from Synechocystis sp. strain PCC 6803 was cloned by heterologous hybridization with the previously isolated icd gene from Anabaena sp, strain PCC 7120. The two cyanobacterial led genes show 81% sequence identity and share a typical 44-amino-acid region different from ail the other icd genes sequenced so far. The led gene scents to be essential for Synechocystis growth since attempts to generate a completely. segregated led mutant were unsuccessful, Transcripts of 2.0 and 1.6 kb were detected by Northern (RNA) blot analysis, for the Anabaena and Synechocystis icd genes, respectively. Maximal icd mRNA accumulation was reached after 5 h of nitrogen starvation in Synechocystis cells and under dinitrogen-fixing conditions in Anabaena cells. Primer extension analysis showed that the structure of the Synechocystis red gene promoter resembles those of the NtcA-regulated promoters. in addition, mobility shift assays demonstrated that purified Synechocystis NtcA protein binds to the promoter of the led gene. All these data suggest that the expression of the icd gene from Synechocystis sp. strain PCC 6803 may be subjected to nitrogen control mediated by the positively acting regulatory protein NtcA.